Ada Reference Manual (Ada 2022) — Legal Information
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4.1.3 Selected Components

1
Selected_components are used to denote components (including discriminants), entries, entry families, and protected subprograms; they are also used as expanded names as described below.

Syntax

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selected_component ::= prefix . selector_name
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selector_name ::= identifier | character_literal | operator_symbol

Name Resolution Rules

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A selected_component is called an expanded name if, according to the visibility rules, at least one possible interpretation of its prefix denotes a package or an enclosing named construct (directly, not through a subprogram_renaming_declaration or generic_renaming_declaration).
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A selected_component that is not an expanded name shall resolve to denote one of the following:
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A component (including a discriminant):
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The prefix shall resolve to denote an object or value of some non-array composite type (after any implicit dereference). The selector_name shall resolve to denote a discriminant_specification of the type, or, unless the type is a protected type, a component_declaration of the type. The selected_component denotes the corresponding component of the object or value. 
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A single entry, an entry family, or a protected subprogram:
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The prefix shall resolve to denote an object or value of some task or protected type (after any implicit dereference). The selector_name shall resolve to denote an entry_declaration or subprogram_declaration occurring (implicitly or explicitly) within the visible part of that type. The selected_component denotes the corresponding entry, entry family, or protected subprogram.
9.1/2
A view of a subprogram whose first formal parameter is of a tagged type or is an access parameter whose designated type is tagged:
9.2/3
The prefix (after any implicit dereference) shall resolve to denote an object or value of a specific tagged type T or class-wide type T'Class. The selector_name shall resolve to denote a view of a subprogram declared immediately within the declarative region in which an ancestor of the type T is declared. The first formal parameter of the subprogram shall be of type T, or a class-wide type that covers T, or an access parameter designating one of these types. The designator of the subprogram shall not be the same as that of a component of the tagged type visible at the point of the selected_component. The subprogram shall not be an implicitly declared primitive operation of type T that overrides an inherited subprogram implemented by an entry or protected subprogram visible at the point of the selected_component. The selected_component denotes a view of this subprogram that omits the first formal parameter. This view is called a prefixed view of the subprogram, and the prefix of the selected_component (after any implicit dereference) is called the prefix of the prefixed view.
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An expanded name shall resolve to denote a declaration that occurs immediately within a named declarative region, as follows: 
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The prefix shall resolve to denote either a package (including the current instance of a generic package, or a rename of a package), or an enclosing named construct.
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The selector_name shall resolve to denote a declaration that occurs immediately within the declarative region of the package or enclosing construct (the declaration shall be visible at the place of the expanded name — see 8.3). The expanded name denotes that declaration. 
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If the prefix does not denote a package, then it shall be a direct_name or an expanded name, and it shall resolve to denote a program unit (other than a package), the current instance of a type, a block_statement, a loop_statement, or an accept_statement (in the case of an accept_statement or entry_body, no family index is allowed); the expanded name shall occur within the declarative region of this construct. Further, if this construct is a callable construct and the prefix denotes more than one such enclosing callable construct, then the expanded name is ambiguous, independently of the selector_name.

Legality Rules

13.1/5
  For a prefixed view of a subprogram whose first formal parameter is an access parameter, the prefix shall be legal as the prefix of an attribute_reference with attribute_designator Access appearing as the first actual parameter in a call on the unprefixed view of the subprogram.
13.2/2
  For a subprogram whose first parameter is of mode in out or out, or of an anonymous access-to-variable type, the prefix of any prefixed view shall denote a variable.

Dynamic Semantics

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The evaluation of a selected_component includes the evaluation of the prefix.
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For a selected_component that denotes a component of a variant, a check is made that the values of the discriminants are such that the value or object denoted by the prefix has this component. The exception Constraint_Error is raised if this check fails.

Examples

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Examples of selected components: 
17/5
Tomorrow.Month     -- a record component                      (see 3.8)
Next_Car.Owner     -- a record component                      (see 3.10.1)
Next_Car.Owner.Age -- a record component                      (see 3.10.1)
                   -- the previous two lines involve implicit dereferences
Writer.Unit        -- a record component (a discriminant)     (see 3.8.1)
Min_Cell(H).Value  -- a record component of the result        (see 6.1)
                   -- of the function call Min_Cell(H)
Cashier.Append     -- a prefixed view of a procedure          (see 3.9.4)
Control.Seize      -- an entry of a protected object          (see 9.4)
Pool(K).Write      -- an entry of the task Pool(K)            (see 9.1)
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Examples of expanded names: 
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Key_Manager."<"    -- an operator of the visible part of a package    (see 7.3.1)
Dot_Product.Sum    -- a variable declared in a function body          (see 6.1)
Buffer.Pool        -- a variable declared in a protected unit         (see 9.11)
Buffer.Read        -- an entry of a protected unit                    (see 9.11)
Swap.Temp          -- a variable declared in a block statement        (see 5.6)
Standard.Boolean   -- the name of a predefined type                   (see A.1)
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